1. Field
The present invention relates generally to equalization in communications systems, and more specifically, to an equalizer with an adaptively adjusting equalizer length for use with wireless communication systems.
2. Background
Communications systems are used for transmission of information from one device to another. Prior to transmission, information is encoded into a format suitable for transmission over a communication channel. The transmitted signal is distorted as it travels through the communication channel; the signal also experiences degradation from noise and interference picked up during transmission.
One effect that creates signal distortion is multipath propagation. Multipath signals are different versions of the same wireless signal that are generated by reflections from structures and natural formations. Multipath signals may have phase shifts that cause the signals to cancel each other out at certain locations. The loss of a signal due to the phase cancellation of multipath signals is known as fading. Fading is a problem in wireless systems because it disrupts user communications. For example, several multipath copies of a single wireless signal transmitted by a wireless communications device may be generated by reflections from trees and buildings. These multipath copies may combine and cancel each other out due to phase offset.
Another issue that may affect a signal is an inadequate signal-to-noise ratio. The signal-to-noise ratio (“SNR”) represents the power of a signal relative to the surrounding noise. An adequate SNR needs to be maintained so that the signal may be separated from the noise.
An example of interference commonly encountered in bandlimited channels is called inter-symbol interference (ISI). ISI occurs as a result of the spreading of a transmitted symbol pulse due to the dispersive nature of the channel, which results in an overlap of adjacent symbol pulses. The dispersive nature of the channel is a consequence of multipath propagation. The received signal is decoded and translated into the original pre-encoded form. Both the transmitter and receiver are designed to minimize the effects of channel imperfections and interference.
Various receiver designs may be implemented to compensate for noise and interference caused by the transmitter and the channel. By way of example, an equalizer is a common choice for dealing with multipath, ISI and for improving the SNR. An equalizer corrects for distortions and generates an estimate of the transmitted symbol. In the wireless environment, equalizers are required to handle time-varying channel conditions. Ideally, the response of the equalizer adjusts to changes in channel characteristics. The ability of the equalizer to respond to changing conditions is related to the number of taps of the equalizer. More taps allow the equalizer to more accurately adjust to changes, while fewer taps allow faster adaptation. Optimizing the equalizer by selecting the number of taps is difficult, as this requires a balancing of competing goals.
A need exists, therefore, for an equalizer design that optimizes performance for a variety of systems and conditions.